This disclosure relates generally to halftoning methods, and more particularly to a halftoning method for producing halftone screens with improved boundary appearance.
In object oriented halftoning, a different halftone screen is assigned to different object types. For example, text objects can be halftoned using a halftone screen which is optimized for text objects; background objects can be halftoned using a halftone screen which is optimized for background objects. Object oriented halftoning has been a long standing attractive idea with the long standing problem of the ragged appearance of the boundaries between adjacent halftones. During object oriented halftoning artifacts are created at the boundaries between two halftone screens. These artifacts give the appearance of ragged edges at boundaries of the printed objects.
The technical reason for edge artifacts is generally related to the different frequency vectors of the adjacent halftone screens.
Some production printers possess the capability to switch halftones on a pixel boundary. Consider an exemplary printer that can use several different halftones that can be switched on a pixel boundary. For printers with this capability, it can be desirable to utilize very low frequency halftones as a solution for achieving a high degree of uniformity in broad tinted areas. A medium frequency halftone can be preferred for image content with a medium degree of edge content. A high frequency screen can be desirable for rendering objects with sharp edges so those edges can be well defined. While the particular halftones may be optimal for a given object type, when different objects are adjacent, such as text on a tinted background, the appearance of the boundary can exhibit the raggedness artifact.
U.S. Pat. No. 5,898,822 to Thomas Holladay for “Using the Phase Information in the Halftone Dot Structure to Minimize Artifacts when Switching Between Halftone Dots on a Scan Line” describes a method of reducing visible image artifacts that uses variable phase parameters (i.e., the standard x, y start position in halftone cells) during halftoning processes to match the phase of halftone cells and also uses a brick approach to halftoning. In Holladay, the position in a halftone dot structure is correlated to the phase of sine waves for subsequent dots to be used in a halftoning process. This method provides some beneficial properties to minimizing image artifacts caused by phase parameters, but it does not address all artifacts, such as the beating artifact at the boundary between objects that occurs in print engines which use low frequency halftone screens, that may result in printers that utilize object oriented halftone printing by adjacent halftone screens.
U.S. patent application Ser. No. 10/909,627, discloses that a method for minimizing boundary effects when switching between halftone screens on a scanline, includes selecting a first halftone screen having a first fundamental frequency and a first angle for printing pixels of a first type; and selecting a second halftone screen having a second fundamental frequency and a second angle for printing pixels of a second type, wherein the second frequency and second angle are harmonically matched to the first frequency and first angle, wherein at least one pixel of the second type is adjacent to a pixel of the first type. In one embodiment, the first and second frequencies are selected to have a substantially zero frequency beat. In another, the first and second frequencies are selected to have a substantially high frequency beat. As such the application addresses those situations of ragged edges where adjacent halftone screens abut. However, this U.S. application Ser. No. 10/909,627, fails to address the situation of ragged edges which only lie upon untoned background such as for the most common example, the white background of a sheet of paper. U.S. application Ser. No. 10/909,627 teaches use of harmonically related screens that produce smooth transitions between two tinted objects. What is needed is a methodology which will handle the edges of tints that may lie on a white background providing them with smooth edges.
Disclosed in embodiments herein is an image processing method for producing image objects with enhanced halftone edges. The method comprises defining border pixels of an image object, selecting a first halftone screen having a first fundamental frequency and a first angle for printing the image object; and selecting a second halftone screen having a second fundamental frequency and a second angle for printing the defined border pixels, wherein the second frequency and second angle are harmonically matched to the first frequency and first angle.
Further disclosed in embodiments herein is a method for minimizing boundary effects by producing enhanced edges for image objects. The method comprises defining border pixels of an image object. The method further comprises selecting a first halftone screen having a first fundamental frequency and a first angle for printing pixels of a first object type for rendering the image object, and selecting a second halftone screen having a second fundamental frequency and a second angle for printing pixels of a second object type for rendering the defined border pixels, wherein the second frequency and second angle are harmonically matched to the first frequency and the first angle.
Further disclosed in embodiments herein is an image processing method for producing image segments with enhanced halftone edges. The method comprises defining border pixels of an image segment; selecting a first halftone screen having a first fundamental frequency and a first angle for rendering the image segment; and selecting a second halftone screen having a second fundamental frequency and a second angle for rendering the defined border pixels, wherein the second frequency and second angle are harmonically matched to the first frequency and first angle.